1. Field of the Invention
The present invention relates to large, stationary turbine power plants and more particularly to a floating seal useful in large, stationary turbine power generators of the type used for utility services.
2. Description of the Prior Art
A typical stationary turbine power plant, known as Model Series 7001 simple cycle, single shaft, heavy duty gas turbine (Frame-7 machine), is available from General Electric of Schenectady, N.Y. In this and similar gas turbines, a seal is located between an axial compressor rotor and a stationary inner barrel member, a chamber within the inner barrel member being supplied with cooling air from the last stage of the compressor by a controlled amount of leakage through the seal. A set of shaft bearings is located in the chamber.
Leakage in excess of a predetermined amount that is required for cooling of the bearings becomes parasitic and contributes to inefficiency of the turbine power plant. This is a serious problem in turbine power plants of the prior art, in that the labyrinth seals, which are positioned to limit the air leakage, degrade in operation because of thermal expansions and other factors that cause knife-edge labyrinth seal members and adjacent rotating elements of the seals to be worn away quickly, particularly during shut-down sequences. This is caused, for example, by shrinkage of the inner barrel member being more rapid than shrinkage of the rotor at the seal. Thus, in the frame-7 machines, the bypass air flow may increase to approximately 100,000 lb/hr or higher from the 30,000 lb/hr that is considered optimal. Consequently, there is a loss of power that is believed to be between 1.5 MW and 3.0 MW.
Brush seals for aircraft gas-turbine engines are known, being disclosed, for example in "Brush Seals", Directions, September 1993. As disclosed therein, a brush seal consists of densely packed metallic bristles that are welded between a down-stream backing plate and an up-stream side plate. In a typical round seal for aircraft turbine applications, the plates are ring-shaped, the bristles extending radially inwardly at a trailing lay angle and making an interference contact with a rotor element, so that the bristles become curved and follow the rotor as it grows and shrinks during engine operation.
Brush seals have not been applied to existing large power plant turbines for a number of reasons. For example, the existing rotor components, being made from elements of low carbon steel alloys that are selected for certain thermal expansion properties, are believed to be unsuitable as wear surfaces for contact by the bristles, particularly during the extended operation cycles that are demanded of stationary power plants. Suitable hardening of applicable compressor rotor members is believed to be prohibitively expensive, particularly in existing equipment.
In U.S. Pat. No. 5,630,590, issued May 20, 1997 to Joseph P. Bouchard and Merrell W. Long entitled "Method and Apparatus for Improving the Air Sealing Effectiveness in a Turbine Engine", a brush seal is disclosed for use in a gas turbine engine. This reference is hereby incorporated in its entirety by reference.
In U.S. Ser. No. 08/892,738, filed Jul. 15, 1997 by Steve Ingistov entitled "Turbine Power Plant Having Minimal-Contact Brush Seal Augmented Labyrinth Seal", Notice of Allowance mailed Apr. 15, 1999, a similar brush seal is disclosed. This application is also incorporated in its entirety by reference.
Both of these references disclose fixed brush seals which are positioned on an end of the inner barrel with suitable spacing from the compressor rotor so that they do not engage existing rotor components. If engagement of the brush seal and the rotor components occurs, the engagement is a result of wobbling of the power shaft connecting the turbine and the compressor, differential expansion of the compressor components and the inner barrel of the machine and the like. As a result, the contact can result in a very high-pressure contact of the brush bristles with the existing rotor components since the bristles are fixed relative to the inner barrel. Accordingly, when contact occurs it can be very forcible, with resulting damage to the rotor components. As a result, clearance space must be provided to attempt to accommodate the worst-case situation. Accordingly, considerable air bypasses the bristles and as a result the use of a labyrinth seal in connection with the bristles is disclosed.
The use of a labyrinth seal in itself entails substantial difficulties with respect to start-up and shut-down operations. These shortcomings are discussed at considerable length in both of the references discussed above.
Since it is desirable, for the reasons discussed above, to control the amount of air passing between the rotor components and the chamber inside the inner barrel, improved sealing methods have been sought.